One type of boiler, which can be found at Brandon Shores Station of Baltimore Gas and Electric Company, is a pulverized coal fired boiler having rows of burners situated on opposing furnace walls, for example, five rows of five burners. Ignitors, identified as "lighters", are installed in each burner. The ignitors are used to warm up the boiler and ignite the pulverized coal flames. Combustion air is distributed to the burners by a compartmented windbox. As generically illustrated in FIG. 1, the burner rows 1 are grouped in compartments 2 with air flows controlled by dampers 3 and measured using air foils 4 at both ends. This design permits balancing of air flows between compartments without changing burner register or vane settings, thus, effectively uncoupling air flow re-distribution between burners from burner aerodynamics.
During start-up, all burner inlet dampers are open and a minimum air flow of 25% of full load air is established. The minimum air flow specification is categorized as a "safe operating practice". It is generally referred to as a purge requirement to flush-out pockets of combustible (even explosive) mixtures of gases from within the boiler enclosure. This practice has been adopted by most utility boiler operations in the U.S. and is based on recommendations from insurance underwriters.
The principal features of the burners are illustrated in FIG. 2. Coal from the pulverizer is transported to the burner in a primary air flow (normally 10-20% of the total combustion air requirement) and is directed into the furnace through a central coal pipe 5. A distributor 6, mounted at the inlet, is intended to minimize flow mal-distributions within the coal pipe. Additional combustion air enters the burners through two cylindrical registers 6.1 outer and 6.2 inner. The register dampers can be rotated from a fully closed to an almost radial direction. The dampers are intended to be used to establish the relative air flows between the inner and outer annular regions of the burner.
A set of "spin vanes" 6.3 are located in the annular space between the coal pipe and the inner register sleeve. These vanes rotate around radial axes and can induce flow directions from clockwise to counterclockwise. The midpoint of the vane's rotation provides axial flow. While the functions of the spin vanes is to provide only enough turbulence to the inner air to establish an ignition zone and maintain stable combustion, their location and design alone provides a means for independently controlling the swirl in the inner annulus while maintaining a desired inner/outer air flow ratio.
The control rods for the registers and spin vanes are connected to levers outside the burner faceplate. The lever positions are set by engaging notches in a fixed plate 6.4. Once determined (during the initial start-up of the unit) the register and vane positions are designed to be kept at these "proper" settings under all operating conditions including; purge, light-off and firing cycles.
As illustrated in FIGS. 3a and 3b, the ignitors consist of an air atomized light oil fired burner 7, a high energy spark probe 8, and a "lighter shield" 9 incorporated into a drive and support assembly 10. A separate pneumatic drive for the spark probe allows the electrode to be retracted after the lighter flame is established. This provision is intended to avoid overheating the high energy electrode. Also shown are a high energy ignitor power supply unit 11, power supply cable 12, atomizing air/steam supply 15, oil supply 16, and oil atomizer 17.
The operating sequence for start-up is unit specific and depends on the configuration of burners and pulverizers and the operating philosophy of the company using the burner. One type of operating sequence for start-up of the ignitors is illustrated in FIG. 4. The critical step in the light-off sequence is the trial for ignition. At the end of this 15 second period the spark probe is de-energized and retracted. At this time all five ignitors in a row must be proven by the flame detectors. If not, the control system terminates ignition and initiates the purge and shutdown sequence. Multiple shut-downs and re-attempts to light and prove lighter flames are a typical occurrence during cold start-ups.
In addition to oil sprays which do not ignite, it is not unusual for the flame detectors to fail to prove an existing flame. FIGS. 3a and 3b, the atomizer 17 is an air-atomized, light-oil, 5 orifice y-jet design. These atomizers produce flames with 5 distinct "fingers". With an 80.degree. spray angle for the atomizer, the distance between flame "fingers" is generally the same as the axial distance from the atomizer at which the flame is viewed. For example, there is a 12-inch gap between flame "fingers" 12 inches from the ignitor. The orientation of the atomizer exit holes with respect to the flame detector is random. Therefore, it is possible that the failure of a flame detector to prove an established flame results from the detector sighting in on the gap between adjacent flame "fingers".
In either case (ignition failures or failure to prove lit flames), approximately 0.4-0.5 gallons of light oil is sprayed into the boiler for each unlit ignitor. A further contribution results from purging fuel from all five ignitors (including those that had been firing). This unburned oil can deposit on boiler surfaces, particularly in the convective passes and the air heater. As temperatures rise, oil retained in the boiler will re-vaporize into the gas flow. Therefore, failures to light and prove ignitor flames, can affect opacity at the time of attempted light-off and for several hours later. Typical opacity levels for cold start-ups are greater than 40% for up to several hours.
In addition to opacity resulting from lighter start-up problems, smoke is consistently observed in the furnace after the lighter flames are established. As shown in FIG. 5 (the opacity chart record for a prior cold start) the combined affects of both mechanisms results in opacity exceeding 10% for approximately 4 hours of the 4 hour and 50 minute period between the start of lighter fuel flow and the energization of the precipitator.